tube cannot be reoriented, added or removed. Lenses come from the factory with a painted-on dot denoting the front face. The inside of the lens tube shows which slot is for which lens. The exception to this is the Source Four Zoom. The Source Four Zoom has a longer barrel, and it has a lens which changes the focal point by moving forward or backward. Different field angles are needed for different venues with different catwalk and electric systems (and, therefore, different throws). A lens tube with a smaller field angle will light an area from far away, whereas a large beam degree such as a 90 degree can be much closer in order to light the same area. A 90 degree Source Four might be used to project a gobo from the rear only 5 feet away on a scrim at the back of the stage, while a 10 degree could be used in the back of the house, for example, in the technical booth where a technician could access it to refocus or change gobos during a show. A zoom gives the option of adjusting the field angle within a specified range without changing the lens tube. This is needed in the case of lighting fixtures that need to be re-focused frequently to different areas of the performing area, without having to rehang them in a different position in the lighting rig. There are two Source Four Zoom fixtures, the 15-30 degree and the 25-50 degree. Enhanced definition lens tube In November 2005, ETC released the Enhanced Definition Lens Tube (EDLT). The EDLT is designed to produce images from gobos and other focus-critical instrument accessories more clearly and accurately than with the standard lens barrels. The lenses in the tube are coated with an anti-reflective material and are machined to more exact standards than the standard Source Four lens. The EDLT also increases lumen output. It is available in 19, 26, 36 and 50 degree barrels. Third party offerings Although the Source Four is designed to accept the same types of accessories as other lighting instruments, when launched in 1992 the smaller lens and barrel size required accessories that fit in a 6.25" holder rather than the 7.5" that was the standard for Lekos at the time. Many third-party manufacturers have designed products specifically for the Source Four. Ocean Thin Films, a manufacturer of scientific optics and instruments,